The present invention relates to high clarity films and articles made from such films. In particular, the invention relates to a blend of metallocene produced polyethylenes (mPE), one of which has a narrow molecular weight distribution and narrow composition distribution, the other has a broader molecular weight distribution and a more narrow composition distribution.
Various types of polymers are used to form films; such films may then be used to form packages or bags. Polymers or blends of polymers used to make films are selected for use because they provide good physical properties, good processability, good clarity or a combination of these attributes.
Since the advent of metallocene catalyzed polyolefins, most especially metallocene catalyzed polyethylenes (mPEs), fabricators of films made from such polyethylenes have sought to find a balance of good physical properties and good optics, along with good processability. Heretofore the physical properties of such polyethylenes have been generally superior to the Zeigler-Natta catalyzed polyethylenes which preceded them. However, melt processing, specifically extruding, has proven more difficult for these later generation polyethylenes (mPEs) versus the earlier Zeigler-Natta catalyzed polyethylenes (Z-N PEs). Further, in both Z-N PEs and mPEs the relative poor clarity of films produced from these polyethylenes has prevented the use of these films in traditional high clarity applications. Such high clarity applications include apparel bags and/or coverings, bread bags, produce bags and the like.
Many plastic materials are known to possess good physical and mechanical properties but often do not also possess good optical properties, specifically of note are the mPEs discussed above. Conversely, some polymers with good clarity do not have adequate strength. Many metallocene catalyzed polyethylenes have excellent physical properties such as dart drop impact and good Elmendorf tear, but have relatively poor clarity and haze.
Blends of high pressure process produced polyethylenes (HP-LDPE) with either mPE and Z-N PE can generally improve the optics of the resulting film over that of either mPE or Z-N PE by themselves. This optics improvement however generally results in a reduction in other physical properties; most notably Elmendorf Tear (generally in the machine direction (MD)) and dart drop impact resistance. Such blends also improve the processing compared to Z-N PE or mPE by themselves.
Previously known blends or coextruded polyethylenes, designed to improve one or more of the properties of a resulting film relative to its components or relative to unblended polyethylene, in the past have also suffered from the drawbacks mentioned above in that improving one property generally leads to diminishing one or more of the other properties. For example, incorporating a blend component with a high average comonomer content to reduce crystallinity and improve heat sealability generally results in an increase of extractables and adversely affects other properties so that the full advantage of the blend is not realized.
U.S. Pat. No. 5,530,065 suggests a blend of a metallocene catalyzed polymer, Component A and a conventional Ziegler-Natta catalyzed polymer, Component B. Component A is said to have a narrow molecular weight distribution and narrow composition distribution, while Component B is said to have a broad molecular weight distribution and a broad composition distribution
Therefore, there is a commercial need for a polyethylene or polyethylene blend that has both good physical properties and excellent clarity and haze.
The blend of polymers of embodiments of this invention generally include at least a first polymer, Component A, which has a narrow molecular weight distribution and a narrow composition distribution and at least a second polymer, Component B, which has a broader molecular weight distribution than Component A and an extremely narrow composition distribution.
In one preferred embodiment of the invention, Component A and Component B is produced by a metallocene catalyst.
In another embodiment of the invention, Component A comprises between 10 to 90 weight percent of the total weight percent polymer blend and Component B comprises between 90 to 10 weight percent of the total weight percent polymer blend of the invention.
The molecular weight distribution or MWD (also Mw/Mn) of Component A is in the range of from 1.5 to 3.0 and the composition distribution breadth index (CDBI) in the range of from 50 to 70%. The relaxation spectrum index (RSI), normalized for melt index of Component A of the blend, will be in the range of from 1.8 to 2.5. The branching factor will be in the range of from 0.95 to 1.0.
The MWD of Component B will be in the range of from 3.5 to 15, and the CDBI in the range of from 75 to 90%. The relaxation spectrum index (RSI), normalized for melt index of Component B of the blend, will be in the range of from 8 to 11. The branching factor will be in the range of from 0.7 to 0.8.
The haze of the blend will be in the range of from 3 to 10%, preferably 4 to 9%, more preferably 5 to 9%, most preferably 5 to 8%, while the gloss (45 degree) will be in the range of from 50 to 80, preferably 55 to 75, more preferably 60 to 75.
In yet another embodiment, the polymer blend of the invention is useful in or as a film or part of a multilayer film in an article of manufacture.